A major difficulty in extracting oil from deposits of heavy, viscous oils or from tar sand deposits results from the poor mobility of the oil in the requisite movement through the deposit and into an oil well. A number of different techniques and apparatus have been developed for reducing the viscosity of the oil by increasing its temperature. In many instances this is accomplished by electrical heating, including particularly conductive heating of a portion of the oil producing formation or "pay zone" adjacent to the well.
In many oil wells it is necessary to perforate that part of a metal well casing that is located in the oil producing formation in order to admit oil into the casing. Customarily, the casing is made of steel pipe. Perforation is usually accomplished by lowering a perforating tool or "gun" into the well casing to the level of the oil producing formation. At that level, the gun fires explosive charges radially outwardly through the casing to form the necessary perforations. Inevitably, this produces a certain amount of debris in and around the well casing, some of the debris constituting sand and other solid particles in the oil deposit that will ultimately find their way into the well. It may also be necessary to employ one or more of various explosive and pressure techniques that fracture the structure of the oil producing formation itself in order to afford convenient and effective passages for the flow of oil from the deposit to the well. Again, these various formation fracturing techniques produce appreciable amounts of sand and other debris which tends to flow to and accumulate in the oil well.
In any of these wells, it may be highly desirable or even essential to provide a rathole. A rathole is a void or space, usually a cased portion of the borehole, that extends generally coaxially of the well bore, to an appreciable distance below the oil producing formation. The rathole affords a deep sump for collecting sand and other solid debris from the perforation and fracturing processes and from other sources, so that this debris cannot accumulate in the well bore immediately adjacent to the pay zone and hence cannot interfere with efficient and effective operation of the well. In many wells the rathole may serve another function, permitting logging instruments or other tools to be positioned in the well below the oil producing formation.
In most electrical well heating systems the steel or other metal casing of the well forms a part of the electrical heating apparatus. If ordinary steel casing were employed in the rathole, as a direct extension of the main well casing, it would be electrically connected or coupled into the heating system and would heat a barren portion of the underburden formations around the well bore below the pay zone. The same situation applies to the metal housing for a float shoe, as usually used in oil wells having cement in the space between the casing and the surrounding formations. Thus, any metal casing that continues downwardly into the rathole and any conductive float shoe housing may represent a substantial source of inefficiency, due to wasted heating of the barren formations surrounding the rathole.